The purpose of this project is to seek experimental evidence to support the recent hypothesis that enzymes obtain the energy needed for catalysis by forming very strong, low-barrier, hydrogen bonds in the transition state, or in otherwise unstable intermediates, while having only weak hydrogen bonds in the ground state. Low-barrier hydrogen bonds have very low field proton NMR chemical shifts, and we will look for such low field signals in enzyme complexes where we think, from other evidence, that they should be present. For example, we will look at fumarase complexed with chlorofumarate, 3-nitrolactate, or 3-nitropropionate, since such complexes should mimic the enolized "carbanion" intermediate in the reaction which we believe must be stabilized by low-barrier hydrogen bonds. Low-barrier hydrogen bonds also have low deuterium fractionation factors, which can be measured by integrating the NMR peaks in mixed H2O and D2O solvent. We will measure the fractionation factors of any low field peaks we find in fumarase complexes, and also the fractionation factors of the low field peaks previously seen in the NMR spectra of aspartate transaminase.